From the past quite some literature is available regarding the extraction of pentose sugars such as xylose and arabinose, from hemicellulose-containing substrates. Xylose thus extracted can be used as such, but it is mainly converted into xylitol or furfural. Arabinose can also be converted into furfural, or further processed into arabitol and/or hydrocracking products. Xylose and/or arabinose can be obtained from a broad range of substrates including by-products obtained during processing of agricultural or forestry raw materials. Typical examples of such by-products are e.g. cereal straw, cereal bran, corn stover, corn cobs, bagasse, sugarbeet pulp, almond shells, coconut shells, or other ligno-cellulosic by-products. This list should not be considered as limiting.
The majority of the prior art processes are using relatively mild acid conditions for hydrolysing hemicellulose, xylan or arabinoxylan structures present in these materials.
The thus obtained pentose solutions may contain quite some mineral and other impurities. When e.g. xylose is used as a substrate for preparing xylitol, these impurities have to be removed in order to obtain xylose solutions which can then be subjected to crystallisation, in view of obtaining high purity xylose.
In order to obtain the required degree of purity, various methods have been described, such as filtration in order to remove particulate impurities, ultrafiltration, ion exchange, decolouring, ion exclusion or chromatography, or combinations thereof (WO2007048879, p. 1, lines 23-27).
As a result of all these purification steps, side streams containing acids, salts and other organic impurities are obtained, which have to be disposed off. As disposal of these side streams becomes more and more expensive, while these side streams are considered as environmentally not desirable, there is a need to limit such side streams, and where possible to re-utilise them in the process.
Several attempts have already been made in order to limit waste streams during refining of xylose/arabinose-containing hydrolysates. Thereby several approaches were used.
One way of reducing waste streams during hemicellulose hydrolysis is by recovering and re-using acid for the hydrolysis of the hemicellulose-containing substrate. In EP219136 acid recovery from a pentose-containing solution is realised by means of a solvent extraction process, while in GB922685 hydrochloric acid is recovered via reduced pressure evaporation of an acidic pentose solution.
In U.S. Pat. No. 5,560,827 and U.S. Pat. No. 5,407,580, a process is disclosed using ion exclusion technology to separate a strong acid from non-ionic components such as sugar. Thereby the acid stream can be re-concentrated and re-used in the hydrolysis step. This reduces the undesirable production of huge quantities of waste gypsum. Strong acid cation exchange resins are used in these processes. The acid stream thus obtained does also contain other positively charged ions. When conducting such a process, there is observed that the separation efficiency between acid and sugars could be subject to improvement with regard to recovery and purity of pentose sugars and acid fractions.
Also in WO9906133 a method is disclosed for separating acid and sugars, obtained from the hydrolysis of ligno-cellulosic material. Thereby ion exclusion chromatography is used to recover and re-use part of the acid, thus reducing acid consumption. In this case a strong base anion exchange resin is used. Thereby, a more pure acid fraction is recovered containing less contaminants such as heavy metals and/or alkali and/or earth alkali metal ions. Compared to acid separation using cation resins in H+ form (U.S. Pat. No. 5,560,827 and U.S. Pat. No. 5,407,580), this option however is less favourable with respect to resin stability.
Recovery of acid also can be realised by means of electro dialysis. Such methods have been disclosed in several patent documents, such as U.S. Pat. No. 5,244,553, CN1477107 and CN101705313.
A third way of recovering acid from a sugar-containing stream is by means of nanofiltration. Such a process is disclosed in U.S. Pat. No. 7,077,953.
Also combinations of technologies are described, thereby focusing on the reduction of waste streams and/or the re-utilisation of acid streams. In the case of CN101392009, an acidic xylose solution is first submitted to a electrodialysis step whereby part of the sulphuric acid is recovered. The remaining stream is then further processed by means of ion exchange (cation/anion exchange sequence followed by mixed bed) and a final nanofiltration step. Thereby applicant states that waste acid and waste alkali are recovered during regeneration of the ion exchange resins, which saves on sulphuric acid consumption and avoids environmental pollution.
In U.S. Pat. No. 4,025,356 a process for the continuous hydrolysis of hemicellulose-containing material is disclosed, whereby at least two portions of the hydrolysate are recycled into the hydrolysis vessel. The first portion is quite concentrated in acid, the second weakly acid portion is obtained by washing the fibrous residue and is re-acidified before introducing it in the continuous hydrolysis vessel. Purpose of the process is to reduce acid consumption.
WO 99/10542 A1 relates to a method of preparing crystalline L-arabinose by extraction of sugar beet pulp, from which sugar has been extracted, in a strong alkaline solution, by hydrolysis of the obtained crude araban with a strong acid at an elevated temperature, by neutralization and filtration of the obtained solution, by chromatographic separation of the L-arabinose fraction, by purification of the obtained L-arabinose solution by means of cation and anion exchangers and adsorbent resins, and by recovering the pure L-arabinose as a crystalline product.
U.S. Pat. No. 4,075,406 describes a method for recovering xylose from pentosan-, preferably xylan-containing raw materials including the steps of hydrolyzing the raw material, purifying the hydrolysate by ion exclusion and color removal, and then subjecting the purified solution to chromatographic fractionation to provide a solution containing a high level of xylose.
Therefore with regard to the acid hydrolysis of pentose-containing polymers present in agricultural and forestry by-products, and the recovery of these pentose sugars, there still is a need for further reducing the formation of waste and the consumption of chemicals during this process. In addition, there is also a need for improving the performance of the processing steps with regard to the recovery of the pentoses and the acid.